This invention relates to a battery charger which suspends charging by detecting the change in battery voltage near the peak voltage, and more particularly to a battery charger that charges while a load is connected.
For the purposes of this application, to "suspend charging" means not only to completely stop charging with a zero charging current condition, but also includes trickle charging where charging current is reduced drastically.
Nickel-cadmium and nickel-hydrogen batteries have the property that battery voltage increases with charging and then decreases by -.DELTA.V from the peak voltage at full charge. Battery chargers utilize this property to detect full charge and cease charging. However, when a load is connected in parallel with the battery during charging, battery voltage drops. Battery charger output current increases and output voltage decreases. The smaller the load resistance, in other words the larger the load current, the larger the drop in battery voltage. Even with large battery charger output impedance, there is a large voltage drop when the load is connected. Note that this situation does not occur for a battery charger that charges a battery after it is disconnected from the electrical equipment load. However, for electrical equipment such as the cordless portable telephone and the laptop computer, the load is connected during battery charging. This is because charging is performed with the battery mounted to the equipment. Particularly in the case of the portable telephone where incoming calls are initiated by another party, the use, cannot determine when the load will be connected. In other words, a decision not to connect the load during charging cannot be made. In addition, since incoming calls on the portable telephone ring a bell in a transient fashion, the load current varies during use. Changes in load current in turn cause changes in battery voltage.
In this manner, in a battery charger for load connection during charging, battery voltage drops when the load is connected. When battery voltage drops, a control circuit judges that the voltage has dropped -.DELTA.V from the fully charged peak battery voltage, and ceases charging. Consequently, battery charging is stopped prior to reaching full charge.
This problem is solved by dividing the power supply into two separate systems, one to charge the battery, and one to supply power to the load. However, this power supply configuration not only complicates the circuitry, but also demands larger power supply output. This is because the required output is the output to charge the battery plus the output to drive the load. In particular, for large load currents, it is necessary to design the power supply to be sufficiently capable of supplying the maximum output for the load.
A power supply can be designed with constant voltage characteristics such that voltage does not drop even when the load is connected. This type of power supply experiences no -.DELTA.V drop when the load is connected during charging. However, the constant voltage power supply also experiences no voltage drop when the battery reaches full charge, and consequently full charge cannot be detected. Therefore, a power supply with constant voltage characteristics cannot be used for the purpose of detecting full battery charge. To detect full battery charge with a single circuit power supply that outputs to a parallel connection of battery and load, characteristics which include a voltage drop when either the load is connected or the battery is fully charged are necessary. In other words, it is a imperative that the power supply have the characteristic that voltage drops when the load is connected. Therefore, a battery charger that senses battery voltage to determine full charge cannot distinguish between connection of the load and full battery charge, and in either case detects a -.DELTA.V drop and ceases charging.
The present invention was developed to solve this problem. It is thus a primary object of the present invention to provide a battery charger for load connection during charging with a simple circuit structure that can fully charge a battery.